Broad band antenna



Amiga 241, 19430 A. ALFORD 2,327,485

BROAD BAND ANTENNA Filed April 12, 1941 IN VENTOH 13w ALFORD A TTORNEY Patented Aug. 24, 1943 Andrew Alford, New Yorlr, N. Y.,

assignor to Mackay Radio and Telegraph Company, New

York, N Y.

a corporation of Delaware Application April 12, 1941, Serial No. 388,246 4 Claims. ((1250-33) This invention relates to antenna systems and more particularly to highfrequency loop antenna having a broad frequency response characteristic. The need for an antenna of simple design having substantially equal response characteristics over wide high frequency band has been realized for some time. However, in past designs it has been diflicult to achieve the desired response characteristic; and compromise systems giving response over a narrower band have of necessity been used. These systems were generally rather complicated in nature and required considerable additional elements to achieve even this partial solution.

According to my invention a simple loop for high frequency waves and having broad band characteristics is provided.

This loop antenna according to my invention, may be useful in many fields and finds particular application for a receiving antenna to be used in instrument navigation of aircraft. A better understanding of my invention and the objects and features thereof may be had from the particular description thereof made with reference to the accompanying drawing, the single figure of which is a diagrammatic illustration of a loop incorporating the features of my invention.

In my prior application Ser. No. 270,173, filed April 26, 1939, Patent No. 2,283,897, granted May 26, 1942, entitled Antenna systems, a relatively simple receiver loop is disclosed. This loop comprises two radiant acting section which are short relative to the wavelengths, these sections being fed at one end and provided at the other end with a terminating transmission line section. 'This loop presents the difficulty that voltage increases toward the ends of the loop and further, that the loop cannot readily be tuned for response to a wide frequency band. Furthermore, the radiation characteristic in the plane of the loop is not circular but is slightly elliptical with the major axis passing through the two current maxima.

According to my present invention I provide a loop made up of four radiant acting sections i, 2, 3, d, sections 2 and 4 being in the order of twice the length of sections I and 3, respectively, and being respectively coupled in series with condensers 5 and t. A further condenser E is provided in series between sections 2 and 4. A tieline comprising conductors 8 and 9 is connected between the remote end of sections I and 3, and the condenser 1. When the antenna is used for transmission, energy is fed from a translator in over a transmission line II to an intermediate mains low in comparison with unity over a point l5, preferably the center, of conductors 8 and 9. When the loop is used for reception of energy translator I 0 may be a receiver.

The particular shape of this loop is variable, it may be circular, elliptical, rectangular, square or an airfoil form.

With this loop construction current maxima occur at intermediate points on conductors 2 and 4 as indicated by the dash-lines l2 and i3. Thus, the distribution is similar to that described in the copending application No. 388,223 for improvement in antenna systems, filed by Melvin A. Rote, on even date herewith. Another current maxima occurs near the junction of conductors l and 3 with lines 8 and a This third current maxima may be slightly intermediate the ends of conductors I or 3, or directly at the junction point or along the tie-line 8, 9, dependent upon adjustment of the elements and the frequency translated by the loop. The current maxima I2 and B3 are located slightly nearer to=condenser 1 than to condensers 6 and 6, and remain relatively fixed in position over the frequency range for which the system is designed.

While the exact nature of the phenomena is not yet completely understood, it is believed that the operation of the circuit is somewhat as follows:

The impedance looking from junction point 15 of line ill and lines 8, 9, toward the condenser I, is resistive and capacitive, while the impedance looking in the opposite direction is resistive and inductive. For this reason at the terminals l5, line H is terminated into something like a parallel tuned circuit consisting of a capacity in series with resistance on one side and inductance in series with resistance on the other side. As the frequency varies the reactive components of the impedances on the two sides of the tuned circuit vary, but remain approximately equal with the result that the impedance looking into junction point remains approximately resistive. Because the resistive components also vary over the frequency range, and namely, in such a manner that the resistance tends to increase when the reactive components tend to increase, the result is that the reflection coefiicient at point I5 rewide band of frequencies.

This antenna may thus be considered as a reentrant circuit consisting of radiant acting elements l, 2, 3,4, and a non-radiating tie-line 8, 9. The translating device is coupled by means of a coupling transmission lin H to a point intermediate the ends of the tie-line, Current distributlon controlling means 5, 6 and I, are provided along the radiant acting elements whereby the impedance at the point of Junction of H and 8, 9 is made substantially constant and resistive over a substantial range of radio frequencies.

In one of the experimental tests the following dimensions were used for the various members of the loop.

The loop had a shape of a square with rounded corners, the radiant acting members consistin of copper tubing 1% in diameter, the sides of the square were 16" long, condenser I had a capacity of approximately 7 micro-micro-farads, and condensers and 6 each had a, capacity of the order of 2 or 3 micro-micro farads. However, the ends of the tubes were close together being separated only about so that there was added capacity between the pipes in addition to the lump capacity of the condensers in parallel with this natural capacity. The tie-line 8, 9 had a surge impedance of 214 ohms and a propagation velocity of .9. Line I! consisted of an approximately quarter-wave section of transmission line with surge impedance of 300 ohms followed by a transmission line of-surge impedance of 214 ohms. This arrangement functioned with a loss due to mismatch not exceeding 1.8 decibels over a range of frequencies from. 109 megacycles to 126 megacycles.

The purpose of the quarter-wave section of 300 ohm line was to more nearly match the impedance at junction'l5 to the 21% ohm line. This was necessary because the impedance at junotion l5 was too high in comparison with 214 ohms and the quarter-wave step-down transformer was used to do the matching. Since the quarter-wave transformer had to be desighed for mean frequency of the band, it is clear that this transformer in itself can produce the band limitation. A somewhat broader band presumably would have been obtained with a high impedance line all the day down to the receiver or with a transformer more aperiodic than the quarter-wave line.

In another experiment the loop antenna consisted of 2 /2" pipes arranged in a square approximately 18 on the sides. In this case capacity l was approximately 11 micro-micro farads, condensers 5 and 6 of the order of 3 micro-micro-farads. This time, as before, there was capacity between the ends of the pipes which were separated about one inch in addition to the capacity of condensers 5 and 6. Thetie-line 8,

9 consisted of a transmission line with 214 ohm surge impedance and with propagation velocity of .9. This time no quarter-wave transformer was used between junction l5 and the 214 ohm line to the transmitter. The impedance at junction 15 was lower in this case and the quarterwave transformer was not required. In this experiment the frequency band extended from 103 megacycles to 125 megacycles with approximately the same loss as in the previous example.

Comparison of the two experiments indicates that the lowering of the surge impedance of the radiantacting conductors by making them of cycles.

larger diameter pipe, leads tosomewhat broader band,-also, the larger radiation resistance obtained with the 19" loop seems to be helpful. The measured radiation resistance of the first loop was approximately 19 ohms at mega- The radiation resistance of the second loop was approximately 32 ohms at 110 megacycles.

While I have described above the features of my invention and some specific experimental models thereof, this description is not to be considered as a limitation on the scope of my invention. From the experimental data it appears that the antenna of this general type will operate over'a wide band of frequencies. It is also clear that various changes and modifications in the radiant acting sections and in the loop can be devised by those skilled in the art within the scope of the teachings of my invention.

What is claimed is:

l. A re-entrant circuit consisting of radiant acting elements and a, non-radiating two conductor tie-line connected between said elements,

a translating device, and a coupling transmission line coupling said translating device to said reentrantcircuit at a point intermediate the ends of said tie-line, and means including. reactive coupling units between said elements for controlling the current distribution along said radiant acting elements to producecurrent loops intermediate the ends of at least some of said elements, whereby the impedance at the point of junction of said coupling transmission line and said tie-line is made substantially constant and resistive over a substantial range of radio frequencies.

2. A broad band high frequency antenna comprising two pairs of radiant acting elements arranged in a substantially closed figure, the elements of one pair being substantially twice the length of the elements of the second pair, one end of each of the longer elements being coupled together through a condenser, the two shorter elements being connected in series with the other ends of the longer elements through substantially equal series condensers, a balanced transmission line interconnecting the free ends of the shorter elements with the ends of the longer elements connected to said first named condenser, said transmission line being transposed, and a feeder transmission line coupled intermediate the ends of said balanced transmission line.

3. A broad band antenna comprising a loop having a plurality of radiant acting sections coupled together with impedance elements including reactance to secure substantially uniform current distribution throughout the loop and a two-conductor tie-line interconnecting certain of the radiant acting elements and means for supplying energy across a point in the tie-line whereby substantial impedance matching over a broad band frequency is obtained.

4. A re-entrant circuit accozding to claim 1, in which said reactive coupling units include capacitive reactance.

ANDREW ALFORD. 

